Rotoform - Realization of Hollow Construction
Elements Through Roto-Forming with
Hyper-Elastic Membrane Formwork
Oliver Tessmann and Samim Mehdizadeh
TU Darmstadt, Digital Design Unit (DDU)
Darmstadt, Germany
{Tessmann, mehdizadeh}@dg.tu-darmstadt.de
ABSTRACT
The paper presents a digital process chain for modeling, simulating and fabricating rotationally molded, individualized hollow concrete components using material-efficient and geometrically flexible formwork systems made from hyperelastic membranes. The hollow concrete components are to be used as prefabricated components for architectural constructions. The inner cavity can be efficient in different ways: To save weight and material, for subsequent filling with other materials (insulating, climate regulating, water heating circulation etc.) or as permanent formwork for solid, reinforced structural components that are poured with concrete. Rotoforming concrete significantly reduces the hydrostatic pressure within a formwork and therefore unlocks completely new possibilities for material-efficient and geometrically flexible formwork systems.
Author Keywords
Complex concrete structures; Casting; Dynamic casting; Membrane formwork; Rotoforming; Minimal surface; Computational design; Simulation; Material behavior; Additive Fabrication.
1 INTRODUCTION
Concrete is one of the most widely used building materials. Mark Wigley conceives of concrete as “the single biggest
form of evidence of our species’s existence” on planet earth
[16] If the material is everywhere it is inevitable to enhance concrete performance i.e. respond to the socio-economic need for a diverse living environment that consumes less material and energy while adapting to various local contexts. Given the ubiquitous use of concrete even minor improvements have a huge impact.
In the construction industry prefab-concrete elements are still bound to a repetitive and serial logic of production. Customized and site specific building parts on the other hand come with high production costs and
material-Computational design allows the simple creation of geometric differentiation. Digital fabrication offers a series of adequate materialization procedures. Data flows fluently from models of ideation and exploration to data for fabrication. This process chain has been tested and established in the timber industry [17]. If, however, it is transferred to concrete structures, a contradiction arises: The promise of concrete taking every possible shape comes with the price of a formwork that supports the material during the process of curing.
This research seeks to bridge the gap that emerges between the possibilities offered by computational design and robotic fabrication and the geometric constraints of conventional formwork systems. This goal is achieved by developing a process chain from digital modeling, physical formfinding, material and process simulation and robotic fabrication. Through migrating the rotomoulding technology into the process of concreting we could reduce the hydrostatic pressure of liquid concrete significantly which allows for a completely new range of lightweight, hyperelastic, compostable membranes as concrete formwork.
2 RESEARCH CONTEXT
Research in the context of fabric formwork, dynamic formwork systems, rotomolding and robotic fabrication is relevant for this project. Fabric formwork is not new .The technology appears in different eras and contexts of the 20th century. The majority of the work is based on craftsmanship. Since the production process, the textiles and the entire formwork setup have a huge impact on the resulting form designers don’t design through drawing but rather by experimentation with scale models and 1:1 prototypes. Veenendaal et al. propose a taxonomy of different textile formwork systems [15]. Within this taxonomy our hyperelastic membrane falls into the category of bi-axial mechanical prestressed formworks. Computational formfinding - the simulation of external
forces impacting on a material system - is only recently migrated into the realm of fabric formwork. The method is known for finding the geometry of form-active structures. In the context of fabric formwork it becomes a construction method [14]. But more important, tools like Kangaroo allow for simulation within an architectural design environment. Thus simulation allows exploring the design potential of fabric formwork before physically making it. The Block Research Group at the ETH Zurich combines tailored fabric formwork with prestressed cable nets as underlying falsework. Concrete is sprayed onto the formwork in thin layer. The cable net deforms under the weight of the sprayed concrete into the shape that is designed by computational methods developed by the researchers. The research thus developed a computational design process for fabric formwork [13]. As the range of possible forms resulting from the use of fabric formwork is limited to shapes that emerge by fabric being exposed to hydrostatic pressure, other research trajectories explore ways to incrementally improve existing formwork. The increase of material efficiency through the use of recycling and re-shaping formwork material such as wax [8] or water [10] is explored in various research projects. Geometric freedom is furthermore achieved through flexible mechanical-kinetic systems such as dynamically reconfigurable double-curved molding surface shaped by an array of actuators [5]. Another example of a dynamic formwork system is the Smart Dynamic Casting (SDC) project by Gramazio Kohler at the ETH Zurich. SDC is based on the concept of slipforming in which concrete is poured into a continuously moving form. The procedure allows for a continuous and gradual change of the cross section of the cast element by shaping the concrete during curing through the subtle movement of the formwork by a robot [6] The project exemplifies the importance of merging design intent, digital fabrication processes and material science into one coherent process. The MARS pavilion by Sarafian, Culver and Lewis exemplifies the use of robots in combination with fabric formwork. The system allows fabricating branching concrete structure cast into adjustable fabric formwork. Robots guarantee the exact position of tailored fabric formwork sleeves that are subsequently assembled into a dome-like lattice structure. The aim was to find a cost competitive way to fabricate parametrically designed concrete structures [9]. Martin Bechthold and Jonathan King from Harvard GSD mass customize concrete objects through robotically orienting a mold while the material cures. The project was presented as a workshop at the Robotics in Architecture 2012 Conference in Graz and Vienna.
None of the mentioned projects addresses the reduction of hydrostatic pressure that we regard as a key concept to unlock a completely new range of material efficient formwork materials. This is achieved by migrating the rotomoulding technology into the realm of concrete processing. Roto-forming is a production process in which
a liquid material is poured into a mold. The amount of material is sufficient to adhere to the wall of the slowly rotating formwork, but not enough to fill the entire mold. The manufacturing process is used in the plastics industry for the production of hollow objects such as water tanks, barrels, kayaks, plastic furniture etc. Here massive steel molds are heated to melt the plastic. Al-Dawery et al migrate rotomoulding from the plastic industry into the field of ceramics [1]. Empirical design and prototyping research on the use of hyperelastic membranes as formwork within a DIY rotomoulding process have first been developed by Thomas Vailly and Itay Ohaly for the production of small-scale design objects. The latex membranes allowed the production of different shapes without the need of previous tailoring [12].
Figure 1. A lightweight hollow concrete (UHPC) object
rotoformed in a prestressed latex membrane.
3 METHOD
3.1 Material System
Rotoforming is conceived of as a material system in which form, material, structure and its synthesis (materialization, fabrication and assembly) are regarded as integral and closely linked elements.
Computational tools and techniques, as part of this system, allow notating and instrumentalizing the intricate interactions between form, material, structure and environment within the architectural design process. Simulating material systems within digital generative models utilizes computation beyond formal and geometrical design schemes. The notion of the model shifts from representation of objects towards the abstraction of a process and the prediction of behavior [3]. The material system approach was used in this project to revisit and challenge conventional formwork systems. Instead of incrementally improving existing formworks we reconsidered the entire process of concreting and identified membranes as formwork material. Besides minimal material consumption these membranes can be tensioned in a wide range of forms without previous tailoring. We reduced the consumption of both concrete and formwork material and at the same time expanded the design potentials for our built environment. This could be achieved through rotoforming concrete.
3.2 The Simulation-Based Design Tool of RotoForm Fabric formwork has no significant tradition in the building industry as it is very different from conventional formwork. A rigid formwork is a technological means to transcend geometry envisioned by the architect into matter. Fabric formwork, in contrast, becomes part of the design process as its material performance and boundary conditions have a significant impact on the resulting shape [14]. The forms that emerge when hydrostatic pressure acts on a fabric or a membrane that is prestressed, can hardly be captured by 2D drawings or even 3D models. Hence, physical models, small-scale or 1:1, have been the tools for designers when working with fabric formwork [7]. Making these physical models and prototypes is a craft that requires a different skill set than the production of drawings or digital models. The craft includes experience and tacit knowledge that is not easy to standardize. As Remo Pedreschi describes, the previously separated roles of builder and the designer merge into one:
"The role of the builder or maker of fabric cast concrete involves both the deconstruction of the object into a sequence of steps and the continual re-evaluation and adjustment of the form during the assembly and casting process. The design develops during the making." [2]
As comprehensible as this coalescence may sound, it is however also responsible for the absence of fabric formwork in the construction industry. Designs that only unfold during making cannot be represented in the conventional artifacts that designers produce. Furthermore design decisions are required during the making, which means that the design phase does not stop with the production of representations such as drawings and models. Designers have to be involved into the materialization.
Against this background we sought to develop RotoForm into a material system consisting of digital and physical components (see Figure 1) that are fluently combined, but also clearly sequenced. Designing with rotoformed elements should be possible without the need for physical prototyping but through the use of simulation-based design tools and methods. Design tools incorporate material behavior under the impact of external forces to overcome mere geometric representation. Thus we use Kangaroo in Grasshopper to simulate the prestressing of membranes. These tools are accessible for designers and well integrated into the architectural design environment. The simulation accomplishes both: It is a technical necessity for the subsequent fabrication, but it also contributes to the representation of the design proposal. The formal potential of the material system is visualized. At the same time its geometric limitations and material constraints are displayed. Digital simulation is not meant to replace prototyping and physical modeling but should rather complement these activities.
4 PROCESS 4.1 Design Process
In this research we conceive of rotoformed elements as nodal connections for irregular space-frame structures. The form of the nodes is a result of a form-finding process in which a particle-spring model is used to find the minimal surface that emerges between all rods intersecting at one point (node) of the space frame. The topology, described by the center-line model, is complemented by a low-polygon solid that approximates the dimensions and orientation of the node (see Figure 3).
Figure 3. Intersecting centerlines, relaxed mesh of nodal
geometry minimal surface.
The mesh resolution is increased for the formfinding simulation using the Grasshopper add-on tool Weaverbird and Meshmachine. Tension forces along the center-lines induce prestressing into the mesh. Mesh edges act as springs and the vertices are exposed to the forces. The particle spring model generates a relaxed mesh approximating the minimal surface that emerges when a hyper-elastic membrane is pre-stressed. Commonly-available software packages, such as Rhinoceros and Grasshopper, Kangaroo are used in order to make the
packages are also used as a means of direct-communication with a UR 10 Universal Robots and a turning table. The simulated form of the nodes is subsequently used to calculate its volume and surface area, two important parameters to the rotoforming process. Based on the surface area and the aspired wall thickness of the hollow element, the amount of cast material is calculated. 4.2 Manufacturing Process
To manufacture the digitally designed and simulated nodes the digital geometry is translated into a prestressed latex membrane kept in place by a spherical falsework (see Figure 7). All steps including their methods and tools are described in the following paragraphs.
Figure 4. Pretentioned the hyper-elastic membrane formwork in
the spherical falsework The adaptive spherical falsework
The pre-stressed membrane is fixed to a spherical falsework that acts as the boundary resisting the large anchoring forces (see Figure 4). The adaptive falsework is designed to allow for the generation of different membrane shapes by changing the position of the tension anchors. Two aluminium plates form the poles of the sphere. The poles are tied together by a series of median arcs (between 6 to 10 depending on geometry). The arcs carry clamps that act as anchoring points and take the loads from tensioning the membrane. 3D printed elements connect the rims to the poles. They can slide in a notch to change the location of the meridian arcs. A screw allows tightening or loosening the connector for proper placement. Clamps that slide along the meridian arcs are holding the anchor rods that define the location and direction for tensioning the membrane.
Robotic placement
A robot translates the digital geometry into the physical setup. In this setup, a UR 10 six axis robot and a turning table are used to place a rod in the correct position and orientation in relation to the sphere. The turntable rotation is controlled via a combination of an Arduino single board micro controller and Funken, a serial protocol toolkit for interactive prototyping [11]. The robotically
positioned rod is fixed to the clamps. After all rods are placed, the membrane formwork is placed inside the falsework. The anchor rods penetrate the membrane and connect it to the falsework. Steel and rubber washers
connect the rods to the membrane and transfer the stresses of the subsequent tensioning.
Figure 5. digital data extraction and Robotic Placement
Pre-tensioning membranes
Membranes are flexible, non-rigid structures that transfer loads through tension. They require fixed ends or rigid linear boundaries that withstand the horizontal forces inherent in every form-active system. Their bearing mechanism relies on the material form. Form coincides with the flow of stresses that are equalized or harmonized along the surface. Loads are dispersed in the direction of resultant forces without any shear [4] In this project we used latex membranes as membrane formwork within the rotoforming process. Filled with air, these balloons take a spherical shape due to internal pressure that acts perpendicular to the membrane surface. Filled with liquid concrete the combination of gravity and hydrostatic pressure generates drop-like shapes. Thus these external forces during production would not allow generating any other form. We therefore sought to minimize this effect through the reduction of cast material in the formwork and through prestressing the membrane formwork (see Figure 6). A series of tension-inducing anchor points are connected to the membrane. In order to achieve a harmonized stress distribution and avoid wrinkles in the membrane the anchor points need to be placed in a way that tension creates curvature in all areas of the membrane.
Figure 6. Pre-tensioning membranes
The Rotoforming machine
The rotoforming machine consists of a frame that rotates around a horizontal spindle powered by an electric motor. The frame carries a vertical spindle to which the spherical formwork is connected (see Figure 7). A belt and 90 degree tapered gear wheels transmit the rotational motion.
Figure 7: The digital-physical robotic-aided process. From left to right: Robotic placement of rods on spherical scaffold, Prestressed membrane formwork. Rotoforming machine.
The movement of the two spindles needs to be aperiodic to make sure that the formwork is fully rotated and all its regions pass the lowest point as the liquid material flows downwards.
The formwork slowly rotates to disperse the liquid cast material to the membrane. In contrast to spun concrete parts, the material is not allocated through centrifugal forces that tend to stratify the material. The material rather adheres to the membrane surface yielding high quality surfaces.
Cast material
During rotoforming the liquid material is subject to a constant change of shape. The impacting loads are not static. In the early stage of curing the material furthermore deforms according to the geometry of the membrane. For this research, the material needed to be adjusted in such a way that it could absorb the quasi-dynamic load at different phases of rotation in different layer thicknesses without cracking. Component sizes, rotational speeds and production speeds as well as the composition of the formwork material have a decisive influence on the viscosity requirements. Mass inertia, adhesive forces on surfaces and hardening processes must be controlled in combination with layer thickness formation and shrinkage cracking.
Two different materials were tested in the project: An ultra high performance concrete (UHPC) and an acrylic/plaster composite. The composite was used for testing the entire process. Its short curing time allows for a fast production of rotoformed elements. However, the main goal is the production of rotoformed concrete elements. Increased hydrostatic pressure and longer curing periods of concrete pose additional challenges to the process. First concrete prototypes were manufactured in collaboration with the concrete company Gtecz (see Figure 1).
Resulting hollow body component
The materialized object is a lightweight hollow component with approx. 3% of the weight of a solid component in
similar size (180 mm radius and 7 mm shell thickness). The resulting surface is smooth with gradually changing curvature continuously blending all directions of the surface. The cast-in anchor rods serve as connection between two components or between nodes and rods. The cast nodes were subsequently 3d scanned in order to compare the cast object to the empty prestressed membranes in the spherical falsework and to the simulated minimal surface of the particle-spring model. The comparison between object and membrane proof that hydrostatic pressure has no impact on defining the shape of the membrane. The simulated form deviates from the form of the cast object. As the current role of simulation is limited to provide a better formal approximation in the early design stage the current precision is satisfactory.
Figure 8. The Section of the resulting demonstrator with hollow
body nodal component.
5 CONCLUSION / OUTLOOK
Within our research we could prove through a novel fabrication process and the resulting prototypes that rotoforming concrete in membrane formwork is possible and leads to material-efficient formworks that allow for geometric differentiation without the need of tailoring the formwork. Furthermore, the process yields hollow concrete objects with reduced weight and material consumptions.
Figure 9. The large scale demonstrator of space truss system at
AAG Conference 2018 with hollow body concrete nodal component
By adding rotational movement to the process of concreting and thereby reducing hydrostatic pressure we are able to reconsider the established palette of formwork materials towards more lightweight and efficient materials. Besides the important aspect of saving resources these materials also simplify the de-molding of concrete objects and generate high surface qualities. The shape of the minimal surface of the prestressed membrane is perfectly mirrored in the concrete object. The tools and methods developed for the process yield geometric precision of the complex forms. A prototype of a series of six interconnected nodes and a rotoformed base plate demonstrate the validity of the material system (see Figure 8 and 9).
Hydrostatic pressure is no longer the form-defining force in this membrane formwork system. However, the need for an even of harmonic stress distribution in the membrane create novel constraints for the range of possible forms. One hypothesis of this research was that such a tool and method would allow designers to explore the formal potentials of membrane formwork and make them part of their design. The hypothesis was tested in a series of workshops with students, researchers and professional designers.
We could observe how the teams implemented material performance into their aesthetic approaches: One example is the design of extra tension-spikes for Harmonizing the stress distribution in the membrane requires similar curvature in all regions of the membrane. A lack of tension in the membrane leads to wrinkles and reduced capacity to withstand the liquid material.
Figure 10. The resulting demonstrator with hollow body
concrete nodal component.
A constraint that may be in conflict with the location and orientation of space frame rods that tension the membrane. When testing the system with students in a rotoforming workshop design team added extra tension-spikes that were independent from the space frame rods, in order to generate the necessary pretension in the system. The integration of simulation of the material system into the early stages of the design process in which the shape is of importance for designers generated novel material-appropriate but also aesthetic design solutions.
The roto-formed elements are significantly lighter than massively cast elements and can thus contribute to more lightweight constructions that consume less material. The reduced weight of a rotoformed facade element allows for more lightweight substructures. The effect thus propagates through the entire construction.
Rotoforming comes with challenges that requires future research in the following fields:
The aim of letting a concrete cure while it is being moved is a conceptual contradiction that can only be solved by the careful design of movements coordinates with concrete recipes that allow for the curing under these delicate circumstances. More data needs to be collected and procedures require standardization to be able to reliably reproduce results of similar quality.
Curing generates heat which is currently collected within the closed membrane system. The heat expands the formwork and can lead to a delimitation of the concrete from the formwork and in the worst case to a collapse of the hollow element during rotation. Casting subsequent layers of material is necessary to not deform the delicate
membrane and the first layers of concrete but requires a cumbersome process of filling the material into the formwork without destroying the previous layer of material.
The above mentioned challenges will be addressed in the ongoing research together with questions of enlarging the range of possible forms and Morphologies through variations of the membrane formwork system and its falsework/boundary condition, the integration of reinforcement and mounting elements and the variation of wall thickness through a differentiated and controlled rotational movement.
AKNOWLEDGMENTS
This research greatly benefited from a series of workshops in which participants tested and prototyped. Their feedback and experiences informed our work. Special thanks to the participants og the workshop at the AAG conference 2018 in Chalmers: Johan Dahlberg, Deena ElMahdy, Eftixis Efthimiou, Felix Graf, Fabio Scotto, Franz Theobald, Athanasios Vagias, Yuwei Zhang. First tests with rotoforming concrete have been conducted with the support of G.tecz/Gregor Zimmermann. The Machines has been built with the technical supports of Mirko Feick/PTU, Marecel Bicolay/ VKM, TU Darmstadt, Alexander Stefas, Andrea Rossi and Felix Graf by DDU of TU Darmstadt. REFERENCES
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